(CNN) Sacramento police officers shot and killed a black man in his grandmother's backyard because they believed he was pointing a gun at them, police said.But investigators say they did not find a weapon at the scene, only a cellphone near the man's body.The fatal shooting of Stephon Clark on Sunday night was recorded by two officers' body cameras and from a police helicopter; that footage was released Wednesday.

The videos show a brief encounter between police and Clark, lasting less than a minute, from the moment one of the officers spotted him in the driveway and yelled, "Hey, show me your hands. Stop. Stop."In the dark, the two police officers chased Clark into the backyard of his grandmother's home."Show me your hands!" one of the officers yelled. "Gun, gun, gun."Then police opened fire. Clark crumpled to the ground, momentarily tried to crawl before falling motionless as more shots erupted around him.

(CNN) Protests erupted in Sacramento four days after Stephon Clark, an unarmed black man, was shot by police in his grandmother's backyard. Demonstrators blocked the entrance to an arena where an NBA game was scheduled Thursday, chanting "Black Lives Matter" and the name of the 22-year-old who was killed.Police said the officers who fired at Clark believed he had a gun. Investigators said they did not find a weapon, only a cell phone.The shooting sparked an outcry in the city, bringing protesters to the Golden 1 Center, where the Sacramento Kings defeated the Atlanta Hawks in a nearly empty arena.

SACRAMENTO — The family wants criminal charges against the officers who opened fire. Protesters are demanding changes in how police approach black men in tense situations. Politicians from the mayor to the lieutenant governor and a U.S. senator express outrage.

But furor aside, there’s little prospect of quick action in the aftermath of the March 18 death of 22-year-old Stephon Clark, an unarmed African American man shot to death in his family’s backyard by two Sacramento police officers.

Experts who study police use of force say the facts and officers’ motives in a fatal shooting are not always immediately clear, despite appearances. Decisions on whether to file charges can take months, even years.[....]“No family should have to endure this pain and suffering as they try to seek answers for an execution of their loved one who was only holding a cell phone,” said Ben Crump, a civil rights attorney who has been retained by Clark’s family. “The Clark family isn’t the only family that has had to endure this suffering.”[....]Clark had two young children. He had several encounters with the law, including convictions for robbery and domestic violence, according to Sacramento County court records, and reportedly had been living in his grandmother’s house for about a month.

Sacramento police shot Stephon Clark seven times from behind, according to autopsy results released Friday by a pathologist hired by Clark's family, findings that call into question the department's assertion that the 22-year-old black man was facing officers and walking toward them when he was killed.

Dr. Bennet Omalu, whose study of a degenerative brain condition in football players prompted the NFL to adopt new safety rules designed to prevent concussions, also determined Clark took up to 10 minutes to die.

Police officers, concerned Clark could be alive and armed, waited about five minutes to approach him after the shooting in his grandmother's backyard. Clark was not armed; police apparently mistook a cellphone in his hand for a gun.

"The proposition is he was facing officers is inconsistent with prevailing forensic evidence," Omalu said at a news conference with family attorney Benjamin Crump.

He said it was not clear if Clark would have survived had he gotten immediate medical attention, and he noted that any of the six bullets that hit him in the back and one in the neck could have been the fatal shot. An eighth bullet went into Clark's thigh.

Sacramento police responded in a brief statement that said the department had not yet received the official report from the Sacramento County coroner's office. It noted that the coroner's death investigation is independent from the investigation being conducted by police and the state Department of Justice.

On Tuesday, the six-month anniversary of the police shooting death of Stephon Clark, a group led by the local chapter of Black Lives Matter will protest outside of the Sacramento Convention Center during a conference of law enforcement officials.

The slate of speakers is scheduled to start at 11 a.m. at a demonstration against fatal police shootings of African Americans — most notably Clark, an unarmed man killed in Meadowview as officers responded to reports of someone breaking car windows.

“We will let them know, not in our streets,” the group said in a news release about the CopsWest Training & Expo, an annual conference of 1,500 officers sponsored by California Peace Officers Association taking place Tuesday and Wednesday.[...]During a news conference for a shooting that left one deputy killed and another injured Monday in Rancho Cordova, Sacramento County Sheriff Scott Jones encouraged others to should head to the convention center to support police.

“We have a planned protest tomorrow at a statewide law enforcement conference down in Sacramento,” Jones said. “I know people have this overwhelming urge to do what they can. ... One of the things you can do is go down there. Go down to the convention center tomorrow between 11 and 1 and show your support for law enforcement.”

Berry Accius, a community organizer involved in the rally, said Jones’ comment was a call to action.

“Scott Jones is declaring war on activists,” Accius said. “ If there is any kind of clash, we know who will be protected and who will be arrested.”